This invention relates to apparatus for making flat or ribbon cables, particularly ribbon cables which comprise a large number of spaced, parallel fine conductor wires encased in insulation material. Wire diameters for this type of cable typically range between 0.010 to 0.020 inches and may include as many as 50 conductors in a ribbon of less than 3 inches.
Ribbon cables are formed in a crosshead having a die which includes a plurality of interconnected die passages for the wires. The crosshead includes a guide block having a plurality of holes axially aligned with the guide passages in the die for guiding the wires into the die. Plasticized or molten insulating compound enters the crosshead from an extruder and flows along appropriate channels to the die in two separate flat streams, one below and one above the array or sheet of wires. As the wires and plasticized insulating material pass through the die, the wires are completely encased to form the finished ribbon cable. The molten insulating material enters the crosshead from the extruder along an axis which is at a substantial angle to the axes of the wires, as much as 90.degree.. Therefore, the flow of the molten or plasticized material must be diverted to the axes of the wires. This creates many problems, since any change in the direction of material flow from a first plane to a second plane that is transverse to the first plane, results in different flow paths for individual cross-sectional increments across the width of the stream of plasticized material. The outer portion of the stream must travel a greater distance that the inner portion. These flow path differences lead to differences in the physical conditions of the plastic compound such as pressure, velocity, temperature, and viscosity. These differences result in non-uniform thickness across the width of the ribbon cable. Because of the great precision required in its use, a non-uniform ribbon cable is unacceptable by the trade in which it is used. In the past, this problem had been partially solved by providing compensating elements such as core pieces or deceleration fittings which alter the flow paths so that the flow path of the material in the inner arc is similar to that in the outer arc.
One mechanism for diverting the flow of insulating material emanating from the extruder from along a first axis to a second transverse axis extending through the die in the crosshead is shown in U.S. Pat. No. 3,758,247 to Deegan, issued Sept. 11, 1973. This patent shows a crosshead die for a ribbon cable in which the flow of insulating material from the extruder is divided into an upper and a lower stream, each of which passes through a helical groove in a diversion roller. The cross-section of each divided stream of insulating material is made rectangular while following along a first path that is generally parallel to the longitudinal axis of the extrude prior to reaching the diversion roller. The helical groove in the diversion roller is rectangular in cross-section and causes the stream of insulating material to be diverted from the first path to a second path that is transverse to the first path and parallel to the array or sheet of conductor wires. The upper and lower streams of insulating material thereafter converge against the wires at the point where the wires enter the die so that the insulating material and wires pass through the die together to form a flat cable.
A major problem encountered in the production of ribbon cable is the clogging of the wire guide holes in the guide block due to the back flow of the plasticized insulating material into the guide block after a wire breaks. The clogging of the wire guide holes prevents reinsertion of a wire into the hole. The broken wire must be removed, the guide hole cleaned and the wire re-inserted through the guide hole into the die. This is extremely difficult to do because of the fineness of the wire. The problem is compounded by the fact that the wire guide holes in the prior art crossheads must be of considerable length because of the space required to divert the flow of material from one axis to another. Eventually, the entire prior art crosshead must be disassembled for cleaning all of the guide holes and the wires re-threaded after assembly.
U.S. Pat. No. 4,150,929 to Brandt, issued Apr. 24, 1979 represents an attempt to solve the clogging problem by providing tubular guide sleeves in the guide block. The guide tubes are necked-down close to the external diameter of the wires near the end of the guide block. However, since the conductor wires are extremely fine and the guide holes approximately 5" long, threading of the wires into the die is very difficult. For a given wire diameter and for a given material, the stiffness of the wires is greatly reduced by increasing the length required to fully insert the wire into the crosshead. Also, most conductor wires are formed from material which is not very stiff to begin with, such as copper. In addition to the difficulties in threading the conductor wires into the crosshead, the use of guide tubes does not entirely eliminate backflow and creates greater problems when the guide tubes become clogged. The guide tubes have to be removed by pliers or the like because the plastic acts to glue the tubes into the crosshead. Since the conductor wires are fine, the guide tubes are also fine and delicate and easily damaged during removal. After removal of the guide tubes, the guide holes must be cleaned and this is still difficult to do because of the narrowness and length of the holes.
None of the prior art crossheads have completely solved the problem of uneven compound flow and clogging up of the guide holes for the wires.
Accordingly, it is the principle object of the present invention to provide a crosshead die for ribbon cable which eliminates the problem of uneven compound flow and minimizes the problem of clogging of the guide holes for the insulating wire.
Another object of the present invention is to provide a crosshead for ribbon cable which greatly facilitates insertion of the wires and is easily cleaned.
A further object of the invention is to provide a crosshead for ribbon cable in which the length of the wire guide holes is greatly reduced, to about 1" long.